Dispersion paints based on polyvinyl acetate, polyvinyl propionate, styrene-butadiene or acrylate-styrene or even solvent-containing paints based on acrylate copolymers are used for painting the surfaces of mineral building materials, particularly when they are used for facades.
The disadvantage of dispersion coatings is that they become soiled relatively quickly and, in many cases, also chalk rapidly, thus resulting in degradation of the paint film.
Paints based on the above-mentioned dispersions and the solvent-containing copolymers have the disadvantage of being poorly resistant to chemicals, particularly solvents. This is an obstacle inter alia in the removal of dirt from walls using aggressive cleaning agents and organic solvents. For these reasons, polyurethane systems have for several years been used to a limited extent for painting facades. The main advantages of polyurethane systems are their high weather resistance, their good surface smoothness, their minimal tendency towards soiling, their high resistance to chemicals and the fact that they are easy to clean.
Light-stable, aliphatic polyurethane paints, however, have the disadvantage of being relatively highly resistant to the diffusion of water vapor (.mu.-factor according to DIN 52 615), with the result that they cannot be used for every type of wall construction (Klopfer: Wassertransport durch Diffusion in Feststoffen, pages 115-122, Bauverlag GmbH, Wiesbaden and Berlin (1975)).
Accordingly, the object of the present invention is to provide polyurethane formulations which show a considerable improvement over conventional polyurethane paints in their permeability to water vapor, so that the paint systems may also be used for coating substrates and wall constructions of the type required to show extremely low resistance to diffusion without the known advantageous properties of polyurethane-based coating compositions or of the coatings produced from them being adversely affected as a result.
Surprisingly, this object may be achieved by using the additives described in more detail hereinafter in dissolved form.